A 1x1 ammo store would be good for tiny mass-driver fighters that are meant to dock. However, I'm not sure that it's worth putting the mass driver on a small fighter like that. Without 4+ accelerators, the mass driver is fairly short range, and it's probably better to go with a fusion cannon.
For me, the real question is whether it's better to put together a fusion canon or just use the plasma lancer. The RoF on the fusion cannon is higher, and the energy use is lower. A cannon that takes up the same 4 tiles as a lancer will do the same damage three times as often, energy permitting, has the same range, and costs only $4K more. The only place where the lancer is the hands-down winner is in projectile speed. The lancer's beam will move at 600m/sec compared to the cannon's 90m/sec. That high speed is a lot better for dealing with ships at speed, but the lack of barrel travel makes the lancer a tough sell.
Thank you, Kroom, for continuing to expand this really awesome mod.
